Underwater wireless optical communication promises to increase data rates beyond those practically achievable using acoustics, with link lengths potentially extending to hundreds of meters. But compared to atmospheric or space-based wireless optical communication, underwater communication faces unique challenges. The absorption spectrum of water dictates that the communication wavelength should be blue or green, rather than the infrared wavelengths used in air and space links. And scattering over short distances in water transfers significant energy outside the beam's diffraction-limited divergence profile, which limits the usefulness of single-mode fiber components at the receiver. Together, absorption and scattering make it challenging to apply some of the technologies (e.g., erbium-doped fiber amplifiers or EDFAs) used in high data rate air and space wireless optical links to underwater wireless optical communication.
In addition, water quality can significantly affect attenuation caused by absorption and scattering. Whereas diffraction-limited air and space links experience attenuation primarily as the inverse-square of distance, loss due to water absorption and scattering is dominated by the Beer-Lambert law:
                              I          ⁡                      (            z            )                          =                                            I              ⁡                              (                0                )                                      ⁢                          e                              -                cz                                              =                                    I              ⁡                              (                0                )                                      ⁢                          e                                                -                                      (                                          a                      +                      b                                        )                                                  ⁢                z                                                                        (        1        )            where a and b represent absorption and scattering coefficients, respectively, c is the overall loss due to the both effects, and z is distance. TABLE 1 (below) shows that the characteristic values for these coefficients vary widely for different several water types. Because of the wide variation in optical loss with water type and optical wavelength, it is useful to measure link distance in terms of extinction lengths, where an extinction length is c−1.
TABLE 1Absorption, scattering, and loss for various water type at different wavelengths. Parameters: a = absorption, b = scattering, c = a + b, EL = extinction length = c−1.WaterabcTypesλ[m−1][m−1][m−1]ELSourceTurbid514 nm0.37 1.8  2.20.45 mPetzold, T.J., “Volume scattering functions forHarbor(green)selected ocean waters,” Scripps Institute ofOceanography SIO 72-78 (1972)Clear514 nm0.11 0.037 0.156.7 mPetzold, T.J., “Volume scattering functions forOcean(green)selected ocean waters,” Scripps Institute ofOceanography SIO 72-78 (1972)470 nm0.038*0.012*0.0520 mPontbriand, C., Farr, N., Ware, J., Preisig, J.,(blue)Popenoe, H., “Diffuse high-bandwidth opticalcommunications,” OCEANS 2008, 15-18September 2008.*Assumed same ratio of b/c as Petzold's clear ocean case.
Scattering at any given location is subject to seasonal variation as suspended biological matter becomes more or less prevalent. Furthermore, scattered sunlight contributes to background noise. Therefore, background noise is subject to variations in water quality, available sunlight at depth, receiver pointing angle, overhead clouds, and time of day.
As can be seen from TABLE 1, signal attenuation over a realistic 10 meter green-light link can vary between 6.6 dB (clear ocean) and 95.5 dB (turbid harbor). Similarly, large variations in link loss can occur for a particular water type if the distance between communicating terminals is changing. Faster changes that could result in deep signal losses (i.e., fades) can be caused by bubbles, turbulence, or large agents, such as fish that appear in the path of the signal.
To compensate for fluctuations in link loss due to changes in link conditions, it is usually necessary to vary the data rate and code rate of the transmitted signal. (At the extreme, it may be necessary to stop transmitting until the link loss drops to acceptable levels.) Typically, these changes must be pre-announced between transmitter and receiver, negotiated within the optical channel using an in-band control channel, or negotiate via a more-reliable side channel. Unfortunately, pre-announced changes are impractical for underwater communications because there aren't any good models of how link loss fluctuates in underwater channels. In-band negotiation is problematic too. If the information rate needs to be reduced because of link impairments, it implies that the link is not reliable enough for the in-band channel to negotiate the change. Out-of-band negotiation using a side channel is also unattractive. In the underwater environment, the most obvious candidate for out-of-band communication is an acoustic channel. However, the acoustic link requires extra hardware. Furthermore, the acoustic signal is much less spatially confined than the optical signal. This spreading is undesirable from an environmental perspective.